Date: Tue, 13 Dec 2016 08:33:34 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
To: Michal Hocko <mhocko@kernel.org>
cc: Andrey Konovalov <andreyknvl@google.com>,
        Felipe Balbi <balbi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Deepa Dinamani <deepa.kernel@gmail.com>,
        Mathieu Laurendeau <mat.lau@laposte.net>, Bin Liu <b-liu@ti.com>,
        USB list <linux-usb@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>,
        Dmitry Vyukov <dvyukov@google.com>, Kostya Serebryany <kcc@google.com>,
        Cristopher Lameter <cl@linux.com>
Subject: Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask
In-Reply-To: <20161213080426.GA10492@dhcp22.suse.cz>
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On Tue, 13 Dec 2016, Michal Hocko wrote:

> > > That being said, what ep_write_iter does sounds quite stupit. It just
> > > allocates a large continuous buffer which seems to be under user
> > > control...  Aka no good! It should do that per pages or something like
> > > that. Something worth fixing
> > 
> > It's not important enough to make the driver do all this work.  If
> > users want to send large amounts of data, they can send it a page at a
> > time (or something like that).
> 
> Is it really necessary to allocate the full iov_iter_count? Why cannot
> we process the from buffer one page at a time?

We could (although one page is really too small -- USB 3.1 can transfer
800 KB per ms so we ought to handle at least 128 KB at a time).  But
turn the argument around: If the user wants to transfer that much data,
why can't he _submit_ it one page at a time?

> > If you really want to prevent the driver from attempting to allocate a
> > large buffer, all that's needed is an upper limit on the total size.  
> > For example, 64 KB.
> 
> Well, my point was that it is not really hard to imagine to deplete
> larger contiguous memory blocks (say PAGE_ALLOC_COSTLY_ORDER). Those are
> still causing the OOM killer and chances are that a controlled flood of
> these requests could completely DoS the system.

Putting a limit on the total size of a single transfer would prevent 
this.

Alan Stern